The Portcullis Gate replaced the ruined 14th century Constable's Tower following the Lang Siege of 1571-3. With two outer doors, a portcullis and an inner door it forms a formidable obstacle. Originally a single storey, the upper level, known as Argyll's Tower, was added in 1584. It is named after Archibald, 9th Earl of Argyll who was imprisoned here in 1685 awaiting execution. The decorative upper work was added in 1886. Above the archway a vaulted chamber houses the portcullis and its winding gear.